(1) Field of the Invention
The present invention relates to a sintered cadmium negative electrode for use in an alkaline storage battery wherein an active material comprising cadmium hydroxide is impregnated in a porous sintered nickel substrate, and a method for producing the sintered cadmium negative electrode.
(2) Description of the Prior Art
In recent years, needs for batteries having an increased capacity and an improved large-current charge-discharge characteristic have been growing in the market of alkaline storage batteries. To meet such needs, various techniques for improving battery performance have been suggested.
For example, as a technique for increasing battery capacity, a method of increasing an impregnating density of an active material in the negative electrode has been suggested. However, such a method increasing an impregnating density of an active material has many drawbacks such as a degradation in an oxygen gas absorbing performance and a deterioration in a charge-discharge characteristic of the negative electrode. Accordingly, it has been difficult to achieve both an increase in capacity and an improvement in battery characteristics such as an oxygen gas absorbing performance and the like.
In view of the above problems, in order to suppress the deterioration in a charge-discharge characteristic of the negative electrode, there has been suggested a technique in which a paste material composed of polyvinyl pyrrolidone (hereinafter referred to as PVP), polyvinyl alcohol (hereinafter referred to as PVA), or polysaccharide such as methylcellulose and starch is coated onto the negative electrode. However, coating such a paste material forms a layer with a low gas permeability on the surface of the electrode, and thereby induces further degradation in an oxygen gas absorbing performance.
Such a degradation in an oxygen gas absorbing performance impairs an excellent large current charge-discharge characteristic of the battery, which is a distinctive advantage in an alkaline storage battery.
The oxygen gas generated from the positive electrode in the event of overcharge is mainly consumed on the surface of the negative electrode by the oxygen gas absorbing reactions as shown below.
(A) 2Cd+O2+2H2Oxe2x86x922Cd(OH)2xe2x80x83xe2x80x83(Chemical oxygen gas absorbing reaction)
(B) 2H2O+O2+4exe2x88x92xe2x86x924OHxe2x88x92xe2x80x83xe2x80x83(Electrical oxygen gas absorbing reaction)
In order to promote such oxygen gas absorbing reactions as shown above, the following methods have been suggested.
(1) By reducing a thickness and increasing a length of the electrode plate, a surface area of the negative electrode is increased to increase a contact area of the oxygen gas and the negative electrode.
(2) As shown in Japanese Patent Publication No. 3-48616, a surface of the electrode plate is brushed with a wire brush and the sintered nickel substrate is thereby exposed, so as to promote the electrical oxygen gas absorbing reaction as shown (B) above.
However, the above-described methods (1) and (2) have the following drawbacks.
Drawbacks with Method (1) Above
When method (1) above is employed, both the chemical oxygen gas absorbing reaction shown as (A) and the electrical oxygen gas absorbing reaction shown as (B) are promoted. However, in order to reduce a thickness of the electrode plate and increase a length of the electrode plate, a relative ratio of the substrate core should inevitably be increased, thereby decreasing the amount of the impregnated active material. In addition, when this method is employed for producing a battery having a spirally-wound power-generating assembly, a diameter of the wound assembly becomes excessively large. As a consequence, method (1) has a drawback that the resulting battery capacity becomes smaller in comparison with a battery with the same size.
Drawbacks with Method (2) Above
When method (2) above is employed, a the electrical oxygen gas absorbing reaction is promoted. However, a surface area of the electrode plate cannot be increased merely by exposing the sintered nickel substrate. As a consequence, method (2) has a drawback that the resulting oxygen gas absorbing reaction as a whole is rendered insufficient.
In view of the foregoing problems and drawbacks of the prior art, it is an object of the present invention to provide a sintered cadmium negative electrode for use in an alkaline storage battery that achieves by increasing an oxygen gas absorbing performance, an improved large current charge-discharge characteristic, as well as an increased capacity.
It is another object of the present invention to provide a method for producing such a sintered cadmium negative electrode for use in an alkaline storage battery that achieves an improved large current charge-discharge characteristic, the method in which no complicated steps are required.
These and other objects are accomplished in accordance with the present invention by providing a sintered cadmium negative electrode for use in an alkaline storage battery comprising a porous sintered nickel substrate and an active material comprising cadmium hydroxide, the active material impregnated in the porous sintered nickel substrate, wherein:
a groove having a depth of 0.1 to 20 xcexcm is provided on a surface of the substrate so that a projected region and a depressed region are provided on the surface of the substrate.
In accordance with the above-described battery construction, a surface area of the electrode plate can be increased without increasing a length of the electrode plate. Therefore, without causing a reduction in a battery capacity, the chemical oxygen gas absorbing reaction and the electrical oxygen gas absorbing reaction are promoted. Moreover, the oxygen gas absorbing performance is improved to a remarkable degree because, unlike the above-described method of exposing the sintered nickel substrate in which only the electrical oxygen gas absorbing reaction is promoted, both the chemical oxygen gas absorbing reaction and the electrical oxygen gas absorbing reaction are promoted.
The depth of the groove is restricted in the range of 0.1 to 20 xcexcm, and this is due to the following reasons. On one hand, if the depth of the groove is made less than 0.1 xcexcm, the resulting surface area of the electrode plate cannot be made sufficiently large, and as a consequence, the improvement in the oxygen gas absorbing performance is rendered insufficient. On the other hand, if the depth of the groove is made more than 20 xcexcm, another problem, a degradation in a strength of the electrode plate, is induced.
In a sintered cadmium negative electrode for use in an alkaline storage battery in accordance with the invention, a polytetrafluoroethylene (PTFE) layer may be formed both on the projected region and on the depressed region.
When a PTFE layer is formed both on the depressed region and on the projected region (i.e., a PTFE layer is formed on the entire surface of the electrode plate), the adhesiveness of the PTFE is increased and thereby a three-layered interface is readily formed on the surface of the electrode plate. Therefore, the oxygen gas absorbing performance is further improved.
Further, in a sintered cadmium negative electrode for use in an alkaline storage battery in accordance with the invention, a PVP layer and/or a PVA layer may be formed on the projected region of the surface of the substrate.
Although a paste material such as PVP and PVA inhibits the oxygen gas absorbing performance, it has an advantageous function of increasing a cycle life of the battery by preventing the aggregation of cadmium caused by repeated charging and discharging. Therefore, as in the above-described construction, when a PVP layer and/or a PVA layer is/are formed only on the projected region of the electrode surface but not on the depressed region, an increase in cycle life can be achieved while a suppression of degradation in the oxygen gas absorbing performance is also achieved.
The foregoing and other objects of the present invention are also accomplished by providing a method for producing a sintered cadmium negative electrode for use in an alkaline storage battery comprising the steps of:
impregnating an active material comprising cadmium hydroxide in a porous sintered nickel substrate to produce an active material-impregnated substrate,
formation-treating the active material-impregnated substrate to produce a formation-treated substrate, and
forming a groove having a depth of 0.1 to 20 xcexcm on a surface of the formation-treated substrate to produce a groove-formed substrate by brushing the formation-treated substrate with a brush so that a projected region and a depressed region are provided on a surface of the formation-treated substrate.
By employing the above-described method, a sintered cadmium negative electrode for an alkaline storage battery such as described above can be readily fabricated.
In a method for producing a sintered cadmium negative electrode for use in an alkaline storage battery in accordance with the invention, the brush may be a wire brush.
By employing a wire brush, a sintered cadmium negative electrode for an alkaline storage battery as described above can be further readily fabricated.
Further, in a method for producing a sintered cadmium negative electrode for use in an alkaline storage battery in accordance with the invention, the wire brush may have a rate of implanted wire of 5 to 10%.
By restricting a rate of implanted wire of the brush, an alkaline storage battery as described above can be further readily fabricated. A rate of implanted wire refers to a rate of an area in a wire brush on which wire is implanted to an entire area in the brush over which wire is distributed.
Further, a method for producing a sintered cadmium negative electrode for use in an alkaline storage battery in accordance with the invention may further comprise a step of forming a PTFE layer both on the projected region and on the depressed region on a surface of the groove-formed substrate by coating or impregnating PTFE on the surface of the substrate, subsequent to the step of forming a groove.
In accordance with such a method, a sintered cadmium negative electrode in which a PTFE layer is formed on the electrode plate can be readily fabricated.
Further, a method for producing a sintered cadmium negative electrode for use in an alkaline storage battery in accordance with the invention may further comprise a step of forming a PVP layer and/or a PVA layer on the surface of the formation-treated substrate by coating or impregnating PVP and/or PVA on the surface of the formation-treated substrate, subsequent to the step of formation-treating.
In accordance with the above method, a sintered cadmium negative electrode in which a PVP and/or a PVA layer is/are formed on the projected region of the electrode plate can be readily fabricated.